While the invention may be applied to a range of applications, consider as an example the manufacture of semiconductor devices such as integrated circuits (ICs) by a lithographic process. In that instance, a lithographic apparatus is used to apply a pattern of device features to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., including part of, one, or several dies) on a substrate (e.g., a silicon wafer).
In lithographic processes, it is desirable frequently to make measurements of the structures created, e.g., for process control and verification. Various tools for making such measurements are known, including scanning electron microscopes (SEM), which are often used to measure properties of a structure, such as the critical dimension (CD). Recently, various forms of scatterometers have been developed for use in the lithographic field. These devices direct a beam of radiation onto a target and measure one or more properties of the scattered radiation as it is reflected and/or transmitted by the target, e.g., intensity at a single angle as a function of wavelength; intensity at one or more wavelengths as a function of angle; or polarization as a function of angle—to obtain a “spectrum” of one form or another. The term “spectrum” in this context will be used with a wide scope. It may refer to a spectrum of different wavelengths (colors), it may refer to a spectrum of different directions (diffraction angles), different polarizations, or a combination of any or all of these. From this spectrum a property of interest of the target can be determined. Compared with SEM techniques, scatterometers can be used with much higher throughput, on a large proportion or even all of the product units. The measurements can be performed very quickly. The time to obtain a measurement result depends on the complexity of the calculations and the processing power available, but can be done off-line. Determination of the property of interest may be performed by various calculation techniques. One particular approach is to perform reconstruction of the target structure by forward modeling of the scattering process, and iterative calculations. In another approach, simulated spectra are calculated in advance for a variety of points in the parameter space. These simulated spectra serve as a “library” which is searched to find a match for a spectrum observed later on a real target.
Frequently the structure is modeled in a parametrized form. Parameters corresponding to the property of interest are considered as “floating” parameters, whose value is (ideally) to be established using the observation data. Other parameters may be fixed. Typically, the property of interest is just one parameter among a number of unknowns, and the model can have many degrees of freedom. Automated methods of optimizing the selection of fixed and floating parameters are described in the prior art, for example in US20120123748.
Examples of known scatterometers include angle-resolved scatterometers of the type described in US2006033921A1 and US2010201963A1. The targets used by such scatterometers are relatively large, e.g., 40 μm by 40 μm, gratings and the measurement beam generates a spot that is smaller than the grating (i.e., the grating is underfilled). In addition to measurement of feature shapes by reconstruction, diffraction based overlay can be measured using such apparatus, as described in published patent application US2006066855A1. If the parameter of interest is an asymmetry-related parameter, such as overlay, a measurement of that parameter can in some cases be obtained relatively directly, based on asymmetry observed in the scatter spectrum. Diffraction-based overlay metrology using dark-field imaging of the diffraction orders enables overlay measurements on smaller targets. Examples of dark field imaging metrology can be found in international patent applications US2014192338 and US2011069292A1 which documents are hereby incorporated by reference in their entirety. Further developments of the technique have been described in published patent publications US20110027704A, US20110043791A, US2011102753A1, US20120044470A, US20120123581A, US20130258310A and US20130271740A. These targets can be smaller than the illumination spot and may be surrounded by product structures on a wafer. Multiple gratings can be measured in one image, using a composite grating target. The contents of all these applications are also incorporated herein by reference.
Prior commercial scatterometers use inspection radiation in the visible and infrared wavebands. As the sizes of features produced by lithography shrink ever smaller and dimensional tolerances shrink accordingly, there is an interest in the use of diffraction based techniques (scatterometry) at shorter wavelengths, such as UV, DUV, “soft x-ray” (extreme ultraviolet) and even x-ray wavelengths. Scattering of electromagnetic waves can be simulated by use of Maxwell's equations at such short wavelengths in the same way as at longer wavelengths. This approach is used to analyze x-ray diffraction patterns from all field of physics: power diffraction, crystallography, biology, etc. In semiconductor manufacturing, reconstruction of critical dimension using small-angle x-ray scattering (CD-SAXS) is already known. Examples are, for example, P. Lemaillet et al, “Intercomparison between optical and x-ray scatterometry measurements of FinFET structures” Metrology, Inspection, and Process Control for Microlithography XXVII, Proc. of SPIE Vol. 8681, 2013 or Ronald L. Jones, et al.“Small angle x-ray scattering for sub-100 nm pattern”, Appl. Phys. Lett. 83, 4059 (2003).
Examples of transmissive and reflective metrology techniques using these wavelengths in transmissive and/or reflective scattering modes are disclosed in pending patent applications PCT/EP2015/058238 filed 16 Apr. 2015, EP15180807.8 filed 12 Aug. 2015 and EP15180740.1 filed 12 Aug. 2015, not published at the present priority date. The contents of all these applications are incorporated herein by reference. In addition to reconstruction and other techniques, asymmetry measurements can be made in scatter spectra, whatever the wavelength.
Conventional techniques naturally assume that the structure under inspection remains constant in shape and composition during an exposure(s) to capture one or more spectra. In the x-ray and EUV range of the electromagnetic spectrum (and sometimes in the DUV, UV and even the visible part of the spectrum) many materials are actually changed to some extent by the inspection radiation. Properties of a material in the structure may change in the course of the observation, and the dimensions of the structure may change. Whenever a property of the structure changes, the measurement of a property of interest may be affected in the same way as a photograph is affected by a subject moving in the course of a long exposure. Changes in one property may influence the result of reconstruction or other measurement techniques, even if the changing parameter is not the parameter of interest.
A well-known example of such changes is the phenomenon of resist shrinkage, which has been observed in lithographic patterning steps performed using EUV radiation. One study derives a model of resist shrinkage, as reported in Peng Liu Leiwu Zheng, Maggie Ma, Qian Zhao, Yongfa Fan, et al. “A physical resist shrinkage model for full-chip lithography simulations”, Proc. SPIE 9779, Advances in Patterning Materials and Processes XXXIII, 97790Y (Mar. 25, 2016); doi:10.1117/12.2239243. The authors in that paper report on modeling of changes occurring in the resist during exposure, prior to development. The paper does not discuss changes that could occur in the resist pattern (or other material), in the course of inspection of the pattern after it has been produced.